1. Field of the Invention:
The present invention relates to a demultiplexing and/or multiplexing optical circuit as a component to constitute a demultiplexer and/or multiplexer which is necessary in a wavelengthdivision multiplexing optical fiber communication system.
2. Description of the Prior Art:
Conventionally, as a demultiplexing and/or multiplexing optical circuit using a buried waveguide, a circuit having a configuration as shown in FIG. 1 is known. (For example, refer to the Institute of Electronics and Communication Engineers of Japan, "Characteristics of Guided-wave Multi-/Demultiplexer with Embedded Waveguides by Ion-Exchange Process", Shingaku giho, OQE86-1, 1986, pp. 1 to 8.)
Referring to FIG. 1, an optica1 waveguide 2 is formed by, e.g., ion exchange method in a transparent substrate 1 made of e.g., glass. The optical waveguide 2 has a higher refractive index than that of the transparent substrate 1. The optical waveguide 2 has an incident path 2A having one end at one side of the substrate 1, a transmitted light output path 2B, and a reflected light output path 2C branching from the incident path 2A at a branching portion 3. A demultiplexing filter 4A which passes (or reflects) a light having a specific wavelength and reflects (or passes) lights having other wavelengths is fitted in the branching portion 3. Bandpass filters 4B and 4C which pass lights having specific wavelengths are fitted in transmitted and reflected light output paths 2B and 2C, respectively.
Grooves 5A, 5B, and 5C are formed in the substrate 1 to cross the optical waveguide 2 at the branching portion 3 and at portions midway along the output paths 2B and 2C. The filters 4A, 4B, and 4C are fitted in the grooves 5A, 5B, and 5C.
Of these filters, the demultiplexing filter 4A is arranged such that the angle defined by the incident path 2A and the reflected light output path 2C is divided into halves by a normal to the surface of the filter 4A.
In the optical circuit having the above arrangement, assume that two lights having different wavelengths .lambda.1 and .lambda.2 are incident on the incident path 2A via an optical fiber or the like. Then, only the light having the wavelength .lambda.1 passes through the filter 4A at the branching portion 3, and lights having wavelengths other than .lambda.1 are rejected by the filter 4B to ensure demultiplexing. The light having the wavelength .lambda.1 is output from the substrate 1 through the transmitted light output path 2B. The light having the wavelength .lambda.2 that has been reflected by the demultiplexing filter 4A at the branching portion 3 is incident to the reflected light output path 2C and is output from the substrate 1 after lights having wavelengths other than .lambda.2 are cut by the filter 4C.
In the conventional demultiplexing and/or multiplexing optical circuit described above, the incident and transmitted light output paths 2A and 2B are formed parallel to a side 1A of the substrate 1. Thus, the groove 5A for receiving the demultiplexing filter 4A is inclined with respect to the side 1A of the substrate 1.
As described above, if the groove 5A must be inclined with respect to the side IA of the substrate 1, when the groove 5A for receiving the filter 4A is formed in a substrate 1, it must be done independently in units of substrates 1 for optical circuits by machining, resulting in a cumbersome operation and poor productivity.
Besides, it is hard to make inclined grooves precisely on a substrate. If the angles of fabricated grooves are not correct, additional loss at branching portions will increase considerably and the yield of demultiplexing and/or multiplexing optical circuits will inevitably decrease, and optical circuits having stable qualities cannot thus be obtained.
It is an object of the present invention to provide a stable-quality demultiplexing and/or multiplexing optical circuit in which the above-described conventional problems are solved, which can be machined easily, and which are thus suitable for mass production.